Gas sensor and method of making

ABSTRACT

A gas sensor is disclosed. The gas sensor includes a gas sensing layer including doped oxygen deficient tungsten oxide and a dopant selected from the group consisting of Re, Ni, Cr, V, W, and a combination thereof, at least one electrode positioned within a layer of titanium, and a response modification layer. The at least one electrode is in communication with the gas sensing layer and the gas sensing layer is capable of detecting at least one gas selected from the group consisting of NO, NO 2 , SO x  O 2 , H 2 O, and NH 3 . A method of fabricating the gas sensor is also disclosed.

BACKGROUND

The invention relates generally to the area of gas sensing. Morespecifically, the invention relates to the sensing of NO_(x) gas.

Environmental considerations are the primary motivating factors todevelop NO_(x) gas sensors. NO_(x) emissions react with gases such asSO_(x), CO and moisture (water vapor) in the air to produce smog andacid rain. One of the major sources of NO_(x) emissions is automobileexhaust.

The European Euro VI emission standards for light commercial vehicles(category N1-I, N1-II and N1-III), to be implemented by September 2015,require NO_(x) emission levels below 0.5 gm/hp-hr. This typicallytranslates to less than 50 ppm of NO_(x) tail pipe emissions.Development of cost-effective gas sensors that can give reliable readoutat such low concentration levels of analyte, and which can deliverrobust performance even in harsh environments, is one of the majorchallenges facing present day emissions monitoring technology.

The current paradigm in improving the efficiency of internal combustionengines utilizes the technology of lean burn, whereby very high air:fuelratios (˜10²:1), as compared to conventional stoichiometric ratio(typically ˜20:1), are used. While the lean burn technology improves theefficiency of the engine, it also results in higher NO_(x) emissions.

Any emissions control scheme that adversely impacts or limits efficiencywill not be commercially viable. This necessitates real time monitoringof NO_(x) emission levels and use of this information to dynamicallycontrol engine operating parameters (such as compression ratio etc) andexhaust after-treatment systems (such as catalytic filters etc) toachieve enhanced engine efficiency and enhanced emissions controlrespectively.

One of the current NO_(x) gas sensing technologies in the market employsyttria stabilized zirconia (YSZ) based gas sensors. The gas sensors areessentially a multi-chamber electrochemical cell measuring the oxygenchanges as a result of NO_(x) decomposition. Such technology requirescatalysts such as Pt. However, the performance of the catalyst degradesupon exposure to SO_(x) and water vapor, as are commonly present in theexhaust from automobiles. This is one of the factors contributing tolowering the working life of such gas sensors. Further, the relativelyintricate design of these gas sensors makes them expensive to replace ona regular basis.

Another current gas sensing technology in the market employssemiconductor gas sensors. As with any technology, this technologypresents situation specific disadvantages and advantages. For example,gas emissions monitoring applications often require quantitativeestimation of a particular or few gas species (e.g., NO_(x)) in amultiple gas species environment. These gas sensors however, aresensitive to a broad range of gases, and therefore are of limitedutility in such applications. Furthermore, these gas sensors are proneto long term instability because of their polycrystalline nature. On theother hand, this technology has the advantages of being solid-state,such as rigid construction and compact size. Further, the technology isamenable to readout using simple electronics thereby reducing cost ofsystem manufacture, operation, maintenance and replacement. In addition,semiconductor gas sensors admit wide range of response tunability viaintroduction of suitable dopants, control of morphology of gas sensingsurface, control of gas sensor operating parameters, amongst othercontrollable factors.

A gas sensor that is semiconductor based, can make quantitativeestimation of NO_(x) gas even at low concentration levels, and have along working life, would therefore, be highly desirable.

BRIEF DESCRIPTION

Embodiments of the invention are directed towards a gas sensor and amethod for making the gas sensor.

In accordance with one exemplary embodiment of the invention, a gassensor is provided. The gas sensor includes a gas sensing layerincluding WO_(3-δ), wherein 0.35≧δ>0 and including a dopant selectedfrom the group consisting of Re, Ni, Cr, V, W and a combination thereof.At least one electrode is positioned within a layer of titanium and aresponse modification layer of a material selected from the groupconsisting of Ti, Ni, Cr, V, W, Re, and a combination thereof, the atleast one electrode being in communication with the gas sensing layer,wherein the gas sensing layer is capable of detecting at least one gasselected from the group consisting of NO, NO₂, SO_(x), O₂, H₂O, and NH₃.

In accordance with another exemplary embodiment of the invention, anautomobile including a system for gas sensing is provided. Theautomobile includes an exhaust system to transport gases, and a gassensor. The gas sensor includes a gas sensing layer including WO_(3-δ),wherein 0.35≧δ>0 and including a dopant selected from the groupconsisting of Re, Ni, Cr, V, W, and a combination thereof. At least oneelectrode is positioned adjacent to a layer of titanium and a responsemodification layer of a material selected from the group consisting ofTi, Ni, Cr, V, W, Re, and a combination thereof, the at least oneelectrode being in communication with the gas sensing layer, wherein thegas sensing layer is capable of detecting at least one gas selected fromthe group consisting of NO, NO₂, SO_(x), O₂, H₂O and NH₃.

In accordance with another exemplary embodiment of the invention, amethod for making a gas sensor is provided. The method includesproviding a substrate, disposing a heating layer adjacent to thesubstrate layer, disposing a first glass layer adjacent to the heatinglayer, disposing a temperature sensing layer adjacent to the first glasslayer, disposing a second glass layer adjacent to the temperaturesensing layer, disposing at least one electrode adjacent to the secondglass layer, disposing a titanium layer adjacent to the at least oneelectrode, disposing a response modification layer adjacent to thetitanium layer, and disposing a gas sensing layer comprising WO_(3-δ),wherein 0.35≧δ>0, and comprising a dopant selected from the groupconsisting of Re, Ni, Cr, V, W and a combination thereof, adjacent tothe titanium layer.

DRAWINGS

These and other features, aspects, and advantages of the presentinvention will become better understood when the following detaileddescription is read with reference to the accompanying drawings in whichlike characters represent like parts throughout the drawings, wherein:

FIG. 1 is a diagrammatical representation of a cross-sectional view of aNO_(x) gas sensor in accordance with an exemplary embodiment of theinvention.

FIG. 2 is a diagrammatical representation of the top view ofinterdigitated sensing electrodes in a NO_(x) gas sensor, in accordancewith an exemplary embodiment of the invention.

FIG. 3 is a diagrammatical representation of the top view of inlinesensing electrodes in a NO_(x) gas sensor, in accordance with anotherexemplary embodiment of the invention.

FIG. 4 is a flow chart representation of a manufacturing process of aNO_(x) gas sensor in accordance with an exemplary embodiment of theinvention.

FIG. 5 is a flow chart representation of a method for detecting theanalyte in accordance with one exemplary embodiment of the invention.

FIG. 6 is a table detailing the effect of oxygen deficiency state (δ) ofa Re doped tungsten oxide film that constitutes the gas sensing layer,when the analyte is NO_(x).

FIG. 7 is a graphical representation of the variation in gas sensorresponse upon exposure to different indicated NO and NO₂ gas levels,wherein the dopant species in a gas sensing layer of the gas sensor isRe, in accordance with one exemplary embodiment of the invention.

FIG. 8 is a graphical representation of the variation in gas sensorresponse upon exposure to different indicated NO and NO₂ gas levels,wherein the dopant species in a gas sensing layer of the gas sensor areRe and Ni, in accordance with one exemplary embodiment of the invention.

FIG. 9 is a graphical representation of the variation in gas sensorresponse upon exposure to different indicated NO and NO₂ gas levels,wherein the dopant species in a gas sensing layer of the gas sensor isCr, in accordance with one exemplary embodiment of the invention.

DETAILED DESCRIPTION

In the following description, whenever a particular aspect or feature ofthe invention is said to comprise or consist of at least one element ofa group and combinations thereof, it is understood that the aspect orfeature may comprise or consist of any of the elements of the group,either individually or in combination with any of the other elements ofthat group.

A gas sensor may be used to determine if an “analyte” is present and/orto quantify an amount of the analyte. As used herein, the term “analyte”may refer to any substance to be detected and/or quantified, includingbut not limited to a gas, a vapor, a bioanalyte, particulate matter, anda combination thereof.

Since the primary constituents of NO_(x), i.e., NO and NO₂ areinterconvertible, reliable estimation of total NO_(x) may be achieved ifthe response of the gas sensor, i.e., the NO_(x) concentration dependentchange in resistance of the gas sensor is equal (in terms of magnitudeand sign) for both NO and NO₂. Thus, if ΔR(NO₂, c) and ΔR(NO, c) be theresponse of the senor to concentration “c” of NO₂ and NO respectively,then a response ratio “rr” (defined below) close to unity would bedesirable.

rr≡ΔR(NO₂, c)/ΔR(NO, c)   (1)

As used herein, the term “equisensitivity” refers to “rr” definedaccording to equation (1) when it is in a range from about 0.5 to about3.

As used herein, the term “adjacent,” when used in context of discussionof different components comprising the gas sensor refers to “immediatelynext to” or it refers to the situation wherein other components presentbetween the components under discussion.

As used herein, the term “communication,” when used in context ofdiscussion of more than one component comprising the gas sensor may meanthat any change in an electrical characteristic of one component isreflected at, and therefore, detectable and measurable via, the othercomponent.

As used herein, the term “harsh environment” or “harsh environments”refers to an environment within a volume that is in the vicinity of thegas sensing layer, and in which are present the analytes whose detectionand/or estimation is being sought. The temperature within this volumemay not be uniform, i.e., the temperature at/of different locationswithin this volume can be different, and can be from about 200° C. toabout 800° C. At different locations within this volume can also bepresent different amounts of corrosive chemical species including butnot limited to NO_(x), SO_(x), H₂O, particulate matter, hydrocarbons,and a combination thereof.

As used herein, the term “response modification layer” refers to a layerwhich serves to introduce dopants into a gas sensing layer via surfacedoping through the mechanism of diffusion. This surface doping mayresult in a modification of the response of the gas sensing layer for agiven set of operating parameters and/or operating environments.

As used herein, the term “glass” refers to any suitable material thatmay be used to form a separating layer, that in a given embodiment ofthe gas sensor, has sufficient thermal conductivity to provide asufficiently large heat link between the elements that the separatinglayer segregates, and which has sufficient electrical resistivity toprovide sufficient electrical resistance between the elements that theseparating layer segregates.

If the response of a particular embodiment of the gas sensor changes asa result of introduction or withdrawal of the analyte, then as usedherein in context of the time of response of the particular embodimentof the gas sensor, the term “fast”, “slow”, and “medium” should beunderstood as follows: let the gas sensor be exposed to a given analytefor a duration “t_(on)” of time, subsequent to which let the analyte bewithdrawn for a duration “t_(off)” of time. Let the response of theparticular embodiment of the gas sensor at the end of “t_(on)” and“t_(off)” be “rf_(on)” and “rf_(off)” respectively. Then the term “fastresponse time” refers typically to the situation where the particularembodiment of the gas sensor achieves at least 0.9 times rf within thefirst minute of exposure to the analyte, i.e., the response of theembodiment of the gas sensor tends to “plateau” off after the firstminute of exposure to the analyte. Another way of saying this is that ananomalous change in time derivative of the response versus time profileoccurs within the first minute or thereabouts of exposure to analyte. Insimilar vein, if the response of the gas sensor does not exhibit aplateau for the entire duration of exposure to analyte then we the term“slow response time” is used to characterize the time response. The term“medium response time” is used to characterize the time response whenthe response of the gas sensor can be characterized as neither “fast”nor “slow”, typically, when the gas sensor achieves, within the firstminute of exposure to analyte, a response of less than 0.5 times rf. Ifthe response of the particular embodiment of the gas sensor changes as aresult of withdrawal of the analyte, then as used herein in context ofthe time of recovery of the particular embodiment of the gas sensor, theterm “fast recovery time” refers typically to the situation where theparticular embodiment of the gas sensor withdraws at least 0.9 times rfwithin about two to three minutes of withdrawal of the analyte. Anotherway of saying this is that an anomalous change in time derivative of theresponse versus time profile occurs within the two or three minutes orthereabouts of withdrawal of analyte. In similar vein, if the responseof the gas sensor does not exhibit any such anomalous change in thepreviously mentioned time derivative up to the entire duration of twentyminutes, then the term “slow recovery time” is used to characterize therecovery time. The term “medium recovery time” is used to characterizethe recovery times when the recovery of the gas sensor can becharacterized as neither “fast” nor “slow”, typically, when it takes thegas sensor response about ten to about fifteen minutes to come to within0.9 times rf.

A gas sensor can be any device capable of producing an electrical signalproportional to an electrical characteristic that can be modulated uponexposure to gases. Examples of suitable devices include, but are notlimited to, a resistor, a field effect transistor, a capacitor, a diode,and a combination thereof.

Examples of suitable gases to be sensed include, but not limited to, NO,NO₂, SO_(x), O₂, H₂O and NH₃ and combinations thereof. In oneembodiment, the gas sensor is not susceptible to poisoning by SO₂ andCO₂ gases.

Referring to the drawings in general and to FIG. 1 in particular, itwill be understood that the illustrations are for the purpose ofdescribing a preferred embodiment of the invention and are not intendedto limit the invention thereto.

FIG. 1 is a diagrammatical representation of one embodiment of a gassensor 100 that may be used to detect an analyte in accordance with anexemplary embodiment of the invention. Although, NO_(x) is used as anexample with respect to some of the embodiments described herein, notethat the gas sensor may be useful to detect other analytes, such as, forexample, SO_(x), NH₃. The gas sensor 100 might be, for example, anin-situ gas sensor that directly samples a gas stream to be analyzed. Inthis way, the sensor 100 can be exposed to the gas stream and generate adetection signal indicating whether a particular analyte (e.g., NO_(x))is present. The gas sensor 100 can also generate a signal proportionalto the concentration of the analyte and thereby measure theconcentration of the analyte.

The illustrated embodiment 100 includes a substrate 102. On thissubstrate layer is disposed a heater 104. A first glass layer 106 ispositioned above the heater. On the first glass layer 106 is disposed atemperature sensing layer 108. A second glass layer 110 is positionedabove the temperature sensing layer. On the second glass layer 110 isdisposed at least one electrode 112. A titanium layer 114 thatcompletely covers the at least one electrode, and which is also incontact with the second glass layer 110 upon which is disposed at leastone electrode. A response modification layer 116 is disposed on to thetitanium layer. Upon this response modification layer is disposed a gassensing layer 118 which has a gas sensing surface 120. In someembodiments, the gas sensor may include an element for heating the gassensor. In one embodiment, an element for heating the gas sensor may bedisposed adjacent to the gas sensing layer or, adjacent to the substratelayer, or on the packaging and any combinations thereof, and/or becovered with an electrically insulating and thermally conducting layer.The heating means may be an element that is separate from the main gassensor, such as a metal (e.g., Pt) layer disposed adjacent to the gassensing layer. In the embodiment illustrated in FIG. 1, the element 104is a 25 Ω(Ohm) Pt heater that may be used to heat the gas sensor to adesired temperature. The heating means may also be the gas sensing layeritself. In one non-limiting example, a large current may be passedthrough the gas sensing layer in order to heat it to a desiredtemperature. The addition of heat, to the surface of the gas sensinglayer may also result in faster response and recovery times. Not to belimited by any particular theory, it is believed that the heat decreasesthe resident time of each gas species at the surface of the gas sensinglayer. The heating means may also allow for adjusting the temperature ofthe gas sensor to allow for higher sensitivity to gas species that arepredisposed to superior detection at higher temperature ranges even whenthe gas stream environment to be measured has not reached suchtemperatures. This may be important in such applications that requiresensing when an engine has only recently been started. Keeping the gassensor at a constant temperature, such as the maximum operabletemperature, can also be used to ignore any dependence of the responsesignal on temperature, thus, allowing for simpler interpretation of theresponse signal. Additionally, the heating of the gas sensor may beintentionally modified to provide a selective response to a variety ofgases as driven by the gas sensing layer temperature dependentselectivity and/or sensitivity to that species of gas. Selectivity asused herein, refers to the ability of a gas sensor to discriminatebetween different presented analyte species. Sensitivity as used herein,refers to the ability of a gas sensor to display a change in anelectrical characteristic when an analyte is presented to it.Selectivity, therefore, may be due to differing sensitivities towardsdifferent analyte species.

In one embodiment, the substrate 102 shown in FIG. 1 may be composed ofAlumina. In another embodiment, the substrate 102 shown in FIG. 1 may becomposed of yttria stabilized zirconia. In yet another embodiment, thesubstrate 102 shown in FIG. 1 may be composed of zirconia.

The glass layers 106 and 110 as shown in FIG. 1, are layers of thermallyconducting but electrically insulating materials that are interposedbetween the heater 104 and temperature sensing layer 108, and betweenthe temperature sensing layer 108 on one side and the at least one ofthe electrode 112 and titanium layer 114 on the other side,respectively. Such glass layers 106 and 110, composed of such thermallyconducting but electrically insulating materials, allow heat to betransported across the gas sensor, yet inhibit electrical contactbetween the heater and the temperature sensing layer or, and between thetemperature sensing layer 108 on one side and the at least one of theelectrodes 112 and titanium layer 114 on the other side, respectively.Examples of suitable materials for glass layers include, but are notlimited to, polysilicate glass, silicon dioxide, silicon nitride, andany combinations thereof. The glass layer 110 may also be subjected tophysical and chemical treatments to enable enhanced physical adhesion ofthe at least one electrode and titanium layer to itself. Further,varying thicknesses of glass layers should allow for different amountsof heat links and electrical resistances between their respectiveenclosing layers.

In one embodiment, the oxygen deficiency (δ) in the host metal oxideWO_(3-δ) can be from about 0 to about 0.5. In another embodiment, theoxygen deficiency (δ) in the host metal oxide WO_(3-δ) can be from about0 to about 0.35.

In one embodiment, the dopant in the gas sensing layer 118 may beselected from the group consisting of Re, Ni, Cr, V, W, and acombination thereof. In another embodiment, the dopant in the gassensing layer 118 can be selected from the group consisting of Re, V anda combination thereof.

In one embodiment, the oxidation state of the dopant may be such thatthe dopant is of n type, e.g., when the dopant is Re in a suitableoxidation state. In another embodiment, the oxidation state of thedopant may be such that the dopant is of p type, e.g., when the dopantis V in a suitable oxidation state. In yet another embodiment, theoxidation state of the dopant may equal the oxidation state of the metal(W) in the metal oxide that the gas sensing layer is composed of, sothat the dopant is neither n type nor p type.

In one example, the response to a given concentration of at least oneanalyte may be enhanced by varying the thickness of the gas sensinglayer. In one embodiment, the gas sensing layer 118 can have a thicknessfrom about 300 Å to about 5000 Å. In another embodiment, the gas sensinglayer 118 can have a thickness from about 500 Å to about 1500 Å. In yetanother embodiment, the gas sensing layer 118 can have a thickness fromabout 700 Å to about 1200 Å.

A response modification layer 116 is interposed between the gas sensinglayer and the titanium layer. This response modification layer iscomposed of a material selected from the group consisting of Ti, Ni, Cr,V, W, Re and a combination thereof. In one embodiment, the responsemodification layer 116 can have a thickness from about 10 Å to about 100Å. In another embodiment, the response modification layer 116 can have athickness from about 20 Å to about 80 Å. In yet another embodiment, theresponse modification layer 116 can have a thickness from about 30 Å toabout 60 Å. In one embodiment of the gas sensor, the responsemodification layer may aid the gas sensor in having an equisensitiveresponse to any two given gases. In another embodiment of the gassensor, the response modification layer may aid the gas sensor in havinga desired value of baseline resistance. In yet another embodiment of thegas sensor, the response modification layer may aid the gas sensor inhaving desired levels of response and recovery times upon exposure toand withdrawal of analyte respectively. Not to be limited by anyparticular theory, it is possible that the response modification layerhelps improve the working characteristics of the gas sensor byinhibiting direct physical contact between the gas sensing layer and thetitanium layer.

In one embodiment, the response modification layer may be composed of amixture of Ti with at least one chemical element selected from the groupconsisting of Ni, Cr, V, W, and Re.

The titanium layer 114 serves as an adhesion layer to anchor theresponse modification layer upon which is disposed the gas sensinglayer. In one embodiment, the titanium layer 114 can have a thicknessfrom about 5 Å to about 100 Å. In another embodiment, the titanium layer114 can have a thickness from about 10 Å to about 50 Å. In yet anotherembodiment, the titanium layer 114 can have a thickness from about 15 Åto about 30 Å.

At least one of the electrodes 112 may be made from any material capableof physical adhesion and electrical contact to its adjacent layers.Examples of suitable materials for the at least one electrode include,but are not limited to, Pt, Au, Ag, Ni, Ti, In, Sn, Cr, nickel nitride,titanium nitride, aluminum doped zinc oxide (ZAO), indium tin oxide(ITO), chrome, and any combination thereof.

In one embodiment, the electrodes 112 can have a thickness from about500 Å to about 10000 Å. In another embodiment, the electrodes 112 canhave a thickness from about 800 Å to about 3000 Å.

In one embodiment, at least one of the at least one electrode 112 may bea multilayer stack of materials. Examples of suitable materials tocomprise the different layers of the multilayer stack include, but arenot limited to, Pt, Ti, Al, Au, Ag, Ni, In, Cr, nickel oxide, titaniumnitride, aluminum doped zinc oxide, indium tin oxide, chrome, and anycombination thereof.

In one embodiment in which at least one of the at least one electrode112 is a multilayer stack of materials, the thickness of each layer canbe from about 100 Å to about 2000 Å. In another embodiment, in which theelectrodes are a multilayer stack of materials, the thickness of eachlayer can be from about 300 Å to about 1500 Å. In yet anotherembodiment, in which the electrodes are a multilayer stack of materials,the thickness of each layer can be from about 500 Å to about 1000 Å.

In one embodiment, the electrodes 112 can be placed in an interdigitatedgeometry 202 as shown in the embodiment 200 in FIG. 2. Element 204 ofFIG. 2 shows the geometry of underlying heater layer 104. In anotherembodiment, the electrodes 112 can be placed in an inline geometry 302as illustrated in the embodiment 300 in FIG. 3. Element 304 of FIG. 3shows the geometry of underlying heater layer 104.

In one embodiment, the at least one electrode may be placed adjacent tothe gas sensing layer in a “sandwich” geometry, i.e., at least oneelectrode is disposed on either side of the gas sensing layer along itsthickness direction. In another embodiment, the at least one electrodemay be placed in a “side-by-side” geometry, i.e., at least twoelectrodes are disposed adjacent each other on the same side of the gassensing layer. In another embodiment, a titanium layer may be placedalong those surfaces of the at least one electrode that are closest toadjacent components of the gas sensor.

In one embodiment, the gas sensing layer 118 can have a dopantconcentration from about 0.2 mol % to about 5 mol %. In anotherembodiment, the gas sensing layer 118 can have a dopant concentrationfrom about 0.5 mol % to about 4 mol %. In yet another embodiment, thegas sensing layer 118 can have a dopant concentration from about 2 mol %to about 3 mol %.

In some embodiments, the gas sensor may include a way of measuring thetemperature of the device. A means of measuring the temperature may bedisposed anywhere within the gas sensor. For example, it may be disposedadjacent to the gas sensing layer or, adjacent to the substrate layer,or on the packaging, or any combination thereof, and/or be covered withan electrically insulating and thermally conducting layer. Thetemperature sensing means may include but not limited to, a resistivetemperature device, a thermocouple, a silicon bandgap temperature sensorand a combination thereof. The temperature sensing means may be aseparate element, such as a metal (e.g., Pt) layer disposed adjacent tothe gas sensing layer.

The temperature sensor can be of various types, including but notlimited to, a thermocouple, a resistance temperature detector, a siliconbandgap temperature sensor, or a thermistor. The thermocoupletemperature sensor can be of various types, including but not limited toType K (CHROMEL®/ALUMEL®), Type J (Iron/Constantan), Type N (NICROSIL®),Type B, Type R, Type S, Type T (Copper/Constantan), Type C. Theresistance temperature detector can be composed of various metals, butare usually made from Pt. The silicon bandgap temperature sensor can becomposed of pure silicon or of chemical compounds of silicon includingbut not limited to silicon carbide. The thermistor temperature sensorcan be composed of various materials including but not limited toceramics and polymers. These materials can have a positive or a negativetemperature coefficient of resistance. The temperature sensors may bebiased in various ways, including but not limited to, voltage biasingand current biasing. Furthermore, the response of the temperaturesensors may be recorded by means including but not limited to resistivemeasurement, potentiometric measurement and a combination thereof. Thetemperature sensor layer 108 shown in FIG. 1 is a thermocouple.

Other gas sensor operating and geometry parameters being fixed, theresponse to a given concentration of any particular analyte may beenhanced when the gas sensing layer is maintained at particulartemperatures. In certain embodiments, enhanced sensitivity may beachieved by maintaining the temperature within the range from about 300°C. to about 550° C.

Conceivably, different applied direct current (DC), alternating current(AC), or a combination thereof, of bias levels to the gas sensing layermay enhance the gas sensing characteristics such as selectivity andsensitivity towards one or another analyte. For example, according tosome embodiments, varying levels of a DC bias may be used to adjust thesensitivity of the gas sensor 100 to different gas species in ananalyte. For example, a gas sensor operating under a given first DC biaslevel might be preferentially sensitive to a first analyte species, agas sensor operating under a second DC bias level might bepreferentially sensitive to a second analyte species, and so on fordifferent DC bias levels. This property may be used to selectivelydetect and measure different species of analyte. The AC and/or DC biasused in the operation of the gas sensor may be an electrical current, anelectrical voltage or a combination thereof. The AC or DC response ofthe gas sensor during operation of the gas sensor may be an electricalcurrent, an electrical voltage or a combination thereof.

The gas sensor may also be configured so as to have suitable one or morefilters that allow only specific analytes to pass through and impingeonto, i.e., make contact, with the gas sensing layer. Conceivably, suchfilters may aid selective detection of given one or more analytes. Suchfilters may also aid in limiting the passage of certain analytes suchas, for instance, particulate matter, towards the gas sensing layer. Insome embodiments, membranes that serve as filters towards particularchemical or physical species present in the environment of the gassensor may be disposed adjacent to the gas sensing layer. Such filterswould provide a means for limiting or regulating the type and/or theamount of gas or particulate species that contact the gas sensing layerof the gas sensor. Examples of suitable means for limiting or regulatingthe type and/or amount of gas species include, but are not limited to, athin film, such as of Kapton, or Teflon, porous membrane filter medium(e.g., steel wool or quartz wool), an about 10 Å thick film of Pd,porous ceramic materials such as Al₂O₃, YSZ, SiO₂, and any combinationsthereof. Conceivably, more than one gas sensor may be placed adjacent toeach other or at different locations within the environment. Each ofthese gas sensors may share with each other the same filter, or mayindividually have one or more, same or different filters. Such an“array” of gas sensors may be used to selectively detect and/or measurethe concentrations of different analyte species at different locationswithin the environment.

FIG. 4 is a flow chart illustrating a method 400 for manufacturing thegas sensor in accordance with an exemplary embodiment of the invention.At step 4O₂, a substrate layer is provided. At step 404, a heater layeris disposed adjacent to the substrate layer. At step 406, a glass layeris disposed adjacent to the heater layer, followed by step 408, where atemperature sensing layer is disposed adjacent to the glass layer. Atstep 410, a glass layer is disposed adjacent to the temperature sensinglayer. At step 412, at least one electrode is disposed adjacent to theinsulating layer. At step 414, a titanium layer is disposed adjacent tothe at least one electrode. At step 416 a response modification layer isdisposed adjacent to the titanium layer. At step 418, a gas sensinglayer is disposed adjacent to the response modification layer.

FIG. 5 illustrates a method for detecting an analyte according to anembodiment of the invention. At step 5O₂, an analyte is allowed toimpinge upon the gas sensing layer of the gas sensor causing a change inan electrical characteristic of the gas sensor. In step 504, the changein the electrical characteristic, which is being monitored continuously,is detected. If the functional relationship between the change in theelectrical characteristic and the concentration of the applied analyteis known, then one may determine the concentration of the appliedanalyte from the measured change in the electrical characteristic, as isshown in step 506. Examples of suitable electrical characteristicsinclude, but a are not limited to, electrical resistance, electricalcapacitance, electrical current, electrical voltage, and a combinationthereof. As an example, if the gas sensor is used under voltage biasconditions, the electrical characteristic might be, for instance,electrical current. Furthermore, such response signals might bemonitored continuously to determine information about time evolution ofconcentration of an analyte.

In one embodiment, the response of the gas sensor, or of the materialcomposing a sensing layer of the gas sensor, may be monitored viaresistive measurement, potentiometric measurement, or combinationsthereof.

In one embodiment, the response of the gas sensor may be tuned to beequisensitive to NO and NO₂ which are the two primary constituents ofNO₁ emissions. The response ratio of the gas sensor for differentconcentrations of NO_(x) may depend upon a plurality of systemparameters and environment parameters, including but not limited to, thelevel of oxygen deficiency in the tungsten oxide film, the one or moredopants that are doped in the tungsten oxide film, the level of dopingof the dopant, the microstructure/morphology of the of the gas sensinglayer, the level of crystallinity of the gas sensing layer, the level ofstrain present in the gas sensing layer, the level of strain present inthe titanium layer, the level of strain present in the responsemodification layer, the temperature at which the gas sensing layer ismaintained while performing the gas sensing, the type and nature of thebias applied across the gas sensing layer, the presence or absence ofthe response modification layer, the level of adhesion of the gassensing layer to the electrodes and to the underlying glass, thematerial, size, design, and placement of the electrodes. Themicrostructure/morphology of the gas sensing layer film may depend onthe method used to grow the film. Some of the above mentioned systemparameters are likely inter-related. In another embodiment, the responseratio of the gas sensor may depend upon the specific set of gas speciesthat are present in the environment and on the individual concentrationsof the different species present. For example, the presence of H₂O inthe environment being sensed may result in a modification in theresponse ratio of the gas sensor.

In one embodiment, the one or more dopants that are incorporated intothe gas sensing layer may aid in modifying one or more responsecharacteristics, including but not limited to, baseline resistance,response time, recovery time, of the gas sensor.

The oxygen deficiency state of the gas sensing layer may be one of theparameters affecting the response of the gas sensing layer. FIG. 6 is atable 600 of the effect of oxygen deficiency state (δ) of a Re dopedhost tungsten oxide film that constitutes the gas sensing layer, whenthe analyte is NO_(x). All the data presented in table 600 were obtainedon gas sensing layers that were formed by the technique of reactivesputtering. The temperature 604 of all the gas sensors was maintained at400° C. The oxygen deficiency state of the host film is an importantparameter that can tune the response ratio of the gas sensor. The oxygendeficiency state of the gas sensing layer is dependent upon severalfactors related to the growth conditions of the gas sensing layer. Thesefactors include, but are not limited to, the pressure of the sputteringgas in the reactive chamber 606, the exact composition of the sputteringgas, i.e., the ratio Ar:O 608, the thickness of the gas sensing layer610, the composition of the targets used 612, and the rate at which thegas sensing layer was deposited 614. The performance of the gas sensors602 may be quantified in terms of the response ratio 616, the time ofrecovery when the analyte is NO 618, and the time of recovery when theanalyte is NO₂ 620. Samples 1 through 7 were grown by co-sputteringtargets composed of WO₃ and Re. The use of a WO₃ target presents acertain minimum amount of oxygen at the location of deposition. Forexample, we see from table 600 that, for samples 1 through 3, at fixedsubstrate temperature 604 and pressure 606, and upon varying the Ar:Oratio 608 from 40:3 to 40:0, the response ratios 616 are nearly thesame, and are substantially less than unity. The recovery times forthese samples towards NO 618 and NO₂ 620 are also not substantiallysensitive to the Ar:O ratio. A substantial reduction in sputtering gaspressure, as well as in the Ar:O concentration, with the expectedconcomitant reduction in the oxygen being presented to thebeing-deposited gas sensing layer, as for samples 4 through 6, also doesnot result in any substantial change in the response characteristics616, 618, 620 from those obtained for the earlier samples 1 through 3.Using now, a W target instead of the earlier WO₃ target, with theconcomitant decrease in the amount of oxygen being presented to thebeing-deposited gas sensing layer, as in for samples 7 through 9,results in substantially modified response characteristics 616, 618,620. The table 600 therefore demonstrates the oxygen deficiency state ofthe gas sensing layer (which has a bearing on the responsecharacteristics of the gas sensing layer) can be tuned by adjusting theoxygen level during the deposition of the gas sensing layer.

In the following measurements presented, the components of the gassensor used may be grouped into two distinct sets depending on theirsource of origin and/or procurement. The five layers, i.e., thesubstrate 102, the heater 104 the first glass layer 106, the temperaturesensing layer 108, and the second glass layer 110 were sourced from acommercial vendor. All other elements, i.e., the electrodes 112, thetitanium layer 114, the response modification layer 116, and the gassensing layer 118, were designed and implemented by the inventors.

The following results of measurements of the response of certainembodiments of the gas sensor were performed according to the followingprotocol: A mixture comprised of gases N₂, O₂, NO, NO₂, SO₂, CO₂, wasfirst prepared by mixing the previously mentioned gases at 300° C. Thismixture gas is then introduced into the chamber where the gas sensor ismounted. The response of the gas sensor, which is maintained at atemperature of about 400° C., is continuously monitored. The exactcomposition of the mixture gas is dependent upon the experiment beingperformed. For example, if the response time of the gas sensor to, say,50 ppm (parts per million) of NO gas has to be ascertained, the mixturegas composition is 1000 sccm of N₂, 100 sccm of O₂, and 50 sccm of 1% NObalanced with N₂. This NO flow is maintained typically, for duration 8or 10 minutes. The response time is obtained from the time evolution ofthe response upon the introduction of the NO gas in to the samplechamber. The recovery time is determined in similar vein by switchingoff of the flow of the NO gas, all other conditions remaining identical.The flow of NO is withdrawn, typically for duration 20 or 30 minutes.The recovery time is obtained from the time evolution of the responseupon the withdrawal of the NO gas in to the sample chamber. Thissequence of steps may be repeated to determine the reproducibility ofthe response.

In one embodiment, the gas sensing layer of the gas sensor may need tobe conditioned before it displays desired and/or adequate response toany given one (or more) analyte(s). For example, when the method ofdeposition of the gas sensing layer is sputtering, then the as-depositedgas sensing layer is likely amorphous. This as-deposited gas sensinglayer may not display desired or adequate response characteristics to,say, NO_(x). Not to be limited by any particular theory, it is believedthat changing the morphology of the gas sensing layer so as to tune itslevel of crystallinity, and/or grain size, and/or grain boundaryinterconnectivity, amongst other factors, will result in improvedresponse characteristics of the gas sensor. It was determined thatannealing the gas sensing layer at high temperatures in the presence ofgases which contain nitrogen and oxygen (e.g., NO_(x)) resulted in thedevelopment, in the gas sensing layer, of desired responsecharacteristics towards NO_(x).

FIG. 7 is a graph 700 illustrating the resistance response of the gassensor over time according to an exemplary embodiment of the inventionwhen the dopant species in the host tungsten oxide film is Re. Themeasurements were performed in two-terminal mode while passing a fixedcurrent equal to 100 nA (nano Ampere) between the electrodes. Differentindicated ppm levels of NO and NO₂ gas were applied successively to thegas sensor. The gas sensing layer for this embodiment of the gas sensorwas deposited via reactive sputtering performed in the followingconditions: the Ar:O pressure was maintained at 13 mTorr, and the Ar:Ogas was made to flow at the rate of 20:13 sccm (standard cubiccentimeter per minute). The gas sensing layer was deposited byco-sputtering of oxygen deficient tungsten oxide and Re targets using RFpower of 240 Watt and 12 Watt respectively. The gas sensing layer wasdeposited at an average rate of about 0.14 Å/s (Angstrom per second) toultimately have a thickness of about 1500 Å. The gas sensor wasmaintained at temperature of about 400° C. when the measurements wereperformed. In this case, the response due to the presence of the firstspecies and second species of analyte (NO₂ and NO respectively) inducesa response (change in resistance) ratio that is, on the average, verynearly 1.7. A response time that is “fast” and a recovery time that is“fast” are demonstrated. Furthermore, the baseline resistance is stableover time, and the response profile is “flat” when the level of analyteis maintained constant.

It has been estimated that the delay time associated with the responseof the gas sensor as the flow of analyte is introduced/withdrawn isexpected to be of the order of few 10¹ s. For example, referring to FIG.7, we estimate the delay time to be less than 30 s.

FIG. 8 is a graph 800 illustrating the resistance response of the gassensor over time according to an exemplary embodiment of the inventionwhen the dopant species in the host tungsten oxide film are Re and Ni.The measurements were performed in two-terminal mode while passing afixed current equal to 250 nA between the electrodes. Differentindicated ppm levels of NO and NO₂ gas were applied successively to thegas sensor. The gas sensing layer for this embodiment of the gas sensorwas deposited via reactive sputtering performed in the followingconditions: the Ar:O pressure was maintained at 4.9 mTorr, and the Ar:Ogas was made to flow at the rate of 21.2:4.0 sccm (standard cubiccentimeter per minute). The gas sensing layer was deposited byco-sputtering of W and Re targets using RF power of 280 Watt and 14 Wattrespectively. The gas sensing layer was deposited at an average rate ofabout 0.75 Å/s (Angstrom per second) to ultimately have a thickness ofabout 1200 Å. The gas sensor was maintained at temperature of about 400°C. when the measurements were performed. The sample was testedcontinuously for a period of over 8500 min. Representative data obtainedover the time period from about 550 min to about 8500 min is presented.In this case, the response due to the presence of the first species andsecond species of analyte (NO₂ and NO respectively) induces a response(change in resistance) ratio that is, on the average, very nearly 2. Aresponse time that is “medium” and a recovery time that is “medium” aredemonstrated. The highly reproducible response of the gas sensor whenexposed to indicated levels of NO_(x) over time, and the highly stablebaseline resistance, demonstrate the excellent working life-time of thegas sensor.

FIG. 9 is a graph 900 illustrating the resistance response of the gassensor over time according to an exemplary embodiment of the inventionwhen the dopant species in the host tungsten oxide film is Cr. Themeasurements were performed in two-terminal mode while passing a fixedcurrent equal to 250 nA between the electrodes. Different indicated ppmlevels of NO and NO₂ gas were applied successively to the gas sensor.The gas sensing layer for this embodiment of the gas sensor wasdeposited via sputtering performed in the following conditions: the Ar:Opressure was maintained at 2.9 mTorr, and the Ar gas was made to flow atthe rate of 15.4 sccm (standard cubic centimeter per minute). The gassensing layer was deposited by co-sputtering of oxygen deficienttungsten oxide and Cr targets using RF power of 240 Watt and 12 Wattrespectively. The gas sensing layer was deposited at an average rate ofabout 1.9 Å/s (Angstrom per second) to ultimately have a thickness ofabout 750 Å. The gas sensor was maintained at temperature of about 400°C. when the measurements were performed. In this case, the response dueto the presence of the first species and second species of analyte (NO₂and NO respectively) induces a response (change in resistance) ratiothat is, on the average, very nearly unity.

In one embodiment, the gas sensor may be used to monitor and/or measurethe concentration of at least one analyte in the exhausts of anautomobile. For instance, the gas sensor may be positioned for enhancedmonitoring and/or measurement of analytes within the exhaust system ofan automobile. In another embodiment, a plurality of gas sensors may bepositioned at different locations within the exhaust system of theautomobile to monitor and measure the concentration of analytes in theexhausts. In another embodiment, a plurality of gas sensors may bepositioned at different locations within the exhaust stream of theautomobile. In another embodiment, the gas sensor may be used to monitorand/or measure the concentration of at least one analyte at otherlocations within the automobile. For instance, the gas sensor may bepositioned for enhanced monitoring and/or measurement of analytes withinthe cabin of an automobile. In another embodiment, a plurality of gassensors may be positioned at different locations within the automobileto monitor and measure the concentration of analytes within the cabin ofthe automobile.

Embodiments of the gas sensor of the present invention may also be usedto monitor emission of NO_(x) in applications including, but not limitedto, aluminum, cement, fertilizer, glass, mineral wool, power, steel,sulphuric acid, and waste incineration industries. In the automobilesector, the gas sensor of the present invention may be used to monitoremissions in a variety of applications including, but not limited to,the emission of NO_(x) from petrol, gasoline and diesel engineautomobiles including, but not limited to, passenger cars, lightcommercial vehicles, lorries, trucks, and buses.

The gas sensor may also be used to meet the U.S. EnvironmentalProtection Agency continuous emissions monitoring standards (CEMS)outlined in 40 C.F.R. §60 and 40 C.F.R. §75. The gas sensor may furtherbe used to meet the European Union CEN emissions limit values. Stillfurther, the gas sensors may be used in a continuous emissionsmonitoring system to determine “cap and trade” allowances as describedby local and federal regulating authorities.

In another aspect, a gas sensor is arranged within an encapsulation in aflip-chip arrangement. In a flip-chip arrangement, the gas sensor isflipped upside down, such that all of the top sensitive surface area ofthe device including the area surrounding the sensitive areas of thedevice, are protected from gases to be monitored. An additionalprotective board protects the back surface of the chip. Directly overthe sensitive area of the device, a slit, or opening in the ceramicboard to which the chip's top surface is mounted, is created to allowthe gases to flow to the gas sensing layer. A layer of high temperaturestable conductive material, such as Pt or Au, may be used tointerconnect the components of the gas sensor to leads in theencapsulation layer. This flip chip arrangement enables interconnect ina higher vibration and higher temperature, for example greater than 500°C., environments than conventional wire bonds, which are susceptible tofatigue failure. The interconnection using platinum and/or gold “bumps”to connect the components, such as the at least one the electrodes tothe leads helps to enable the use of the gas sensor in harshenvironments.

In one embodiment the gas sensor may be configured to be operable inharsh environments in which are present locations where the temperatureis between about 200° C. and about 800° C. In another embodiment, thegas sensor may be configured to be operable in harsh environments inwhich are present locations with temperature is between about 200° C.and about 600° C. In yet another embodiment, the gas sensor may beconfigured to be operable in harsh environments in which are presentlocations with temperature is between about 300° C. and about 500° C.

The gas sensor is cost effective in that it has a long working life(˜10³ hours) and provides highly repeatable readout. The costeffectiveness is further enhanced because of the simple modular designof these gas sensors allowing ready scaling of the manufacturing processto large volumes.

The gas sensor may be encapsulated in a packaging. The encapsulationfurther protects the gas sensor from the high temperature and corrosiveatmosphere in the harsh environments where these gas sensors are likelyto be used. The encapsulation acts to cover exposed surfaces of suchelements of the device as the titanium layer, the electrodes, the firstglass layer, the thermometer, the second glass layer, the heater, andthe substrate, which do not by themselves, sense the gases. Thisencapsulation may also involve forming a bond with the underlying layer(substrate), so as to not permit flow of gases and corrosive materials(e.g., particulate matter, hydrocarbons) that would be detriment to thedevice over time. Examples of such suitable materials for encapsulationinclude, but are not limited to, silicon carbide, ceramic based epoxiessuch as those containing alumina, glass, quartz, silicon nitride,silicon dioxide and a combination thereof.

The encapsulation layer can be deposited by any known method, such asplasma enhanced chemical vapor deposition (PECVD), low pressure chemicalvapor deposition (LPCVD), and a combination thereof. The encapsulationis such that at least a portion of the gas sensing layer remains exposedto ambient gases. With the application of an encapsulation the gassensors may be protected in harsh environments and have a longer workinglife. Such protection against harsh environments would allow for the useof these sensors in a wide variety on settings, including but notlimited to, boiling water reactor, automotive and locomotive petrol ordiesel engine exhaust, industrial process (glass, aluminum, steel, andpetroleum) plant exhaust. It would further protect the gas sensor fromthe particulate matter that may be present in the exhaust streams of thepreviously mentioned environments. Such particulate matter maypotentially be detriment to the gas sensor as they may adhere to and/orcorrode the gas sensor thereby hindering the detection of exhaust gasesby hindering contact between the exhaust gases and the gas sensor.

While the invention has been described in detail in connection with onlya limited number of embodiments, it should be readily understood thatthe invention is not limited to such disclosed embodiments. Rather, theinvention can be modified to incorporate any number of variations,alterations, substitutions or equivalent arrangements not heretoforedescribed, but which are commensurate with the spirit and scope of theinvention. Furthermore, while single energy and dual-energy techniquesare discussed above, the invention encompasses approaches with more thantwo energies. Additionally, while various embodiments of the inventionhave been described, it is to be understood that aspects of theinvention may include only some of the described embodiments.Accordingly, the invention is not to be seen as limited by the foregoingdescription, but is only limited by the scope of the appended claims.

1. A gas sensor, comprising: a gas sensing layer comprising WO_(3-δ),wherein 0.35≧δ>0, and comprising a dopant selected from the groupconsisting of Re, Ni, Cr, V, W, and a combination thereof; and at leastone electrode positioned adjacent to layer of titanium and a responsemodification layer of a material selected from the group consisting ofTi, Ni, Cr, V, W, Re and a combination thereof, the at least oneelectrode being in communication with the gas sensing layer; wherein thegas sensing layer is capable of detecting at least one gas selected fromthe group consisting of NO, NO₂, SO_(x), O₂, H₂O and NH₃.
 2. The gassensor of claim 1, comprising: a substrate layer; a heating layeradjacent to the substrate layer; a first glass layer adjacent to theheating layer; a temperature sensing layer adjacent to the first glasslayer; a second glass layer between the temperature sensing layer andthe titanium layer.
 3. The gas sensor of claim 1, wherein the gassensing layer is configured for equisensitive response to two givengases.
 4. The gas sensor of claim 1, wherein the gas sensing layer isconfigured to be operable in harsh environments.
 5. The gas sensor ofclaim 1, wherein an analyte species filter material is disposed adjacentto the gas sensing surface of the gas sensing layer.
 6. The gas sensorof claim 1, wherein the at least one electrode is composed of a materialselected from the group consisting of Pt, Au, Ag, Ni, Ti, In, Sn, Cr,nickel oxide, titanium nitride, aluminum doped zinc oxide, indium tinoxide, and a combination thereof.
 7. The gas sensor of claim 1, whereinthe at least one electrode is composed of a multilayer stack ofmaterials selected from the group consisting of Pt, Ti, Al, Au, Ag, Ni,Cr, In, titanium nitride, nickel oxide, aluminum doped zinc oxide,indium tin oxide, chrome, and a combination thereof.
 8. The gas sensorof claim 1, wherein the at least one electrode is placed in a sandwichgeometry, a side-by-side geometry, or combinations thereof.
 9. The gassensor of claim 1, wherein the gas sensing layer has a response timefrom about 1 s to about 100 s upon exposure to analyte.
 10. The gassensor of claim 1, wherein the gas sensing layer has a recovery timefrom about 1 s to about 100 s after exposure to analyte is withdrawn.11. The gas sensor of claim 1, wherein said at least one electrodecomprises at least two electrodes and an electrical resistance betweensaid at least two electrodes is less than about 100000Ω.
 12. The gassensor of claim 1, wherein the at least one electrode can be placed inan interdigitated geometry, an inline geometry, or combinations thereof.13. The gas sensor of claim 1, wherein the gas sensor is arranged in aflip-chip arrangement.
 14. The gas sensor of claim 1, wherein the gassensing layer is configured for detection of analyte levels from about 1ppm to about 1000 ppm.
 15. The gas sensor of claim 1, wherein the gassensor is configured to be operable as a resistor, a field effecttransistor, a capacitor, a diode, and a combination thereof.
 16. The gassensor of claim 1, wherein the gas sensor is configured so that itsresponse may be monitored via resistive measurements, potentiometricmeasurements, or combinations thereof.
 17. The gas sensor of claim 1,wherein the electrode thickness is from about thickness 500 Å to about10000 Å.
 18. The gas sensor of claim 1 when the at least one electrodecomprises a multilayer stack of materials, wherein a thickness of eachlayer of the multilayer stack is from about 100 Å to about 2000 Å. 19.The gas sensor of claim 1, wherein the response modification layerthickness is from about 10 Å to about 100 Å.
 20. The gas sensor of claim1, wherein the titanium layer thickness is from about 5 Å to about 100Å.
 21. The gas sensor of claim 1, wherein the gas sensing layerthickness is from about 300 Å to about 5000 Å.
 22. The gas sensor ofclaim 1, wherein the concentration of the dopant in the gas sensinglayer can be from about 0.2 mol % to about 5 mol %.
 23. The gas sensorof claim 1, wherein the response of the gas sensor may be measured usingan AC detection technique, a DC detection technique, or a combinationthereof.
 24. A gas sensor array, wherein a plurality of gas sensors ofclaim 1 are placed adjacent to each other.
 25. An automobile having asystem for gas sensing, comprising: an exhaust system to transportgases; and a gas sensor, comprising: a gas sensing layer comprisingWO_(3-δ), wherein 0.35≧δ>0, and comprising a dopant selected from thegroup consisting of Re, Ni, Cr, V, W, and a combination thereof; atleast one electrode positioned within a layer of titanium and a responsemodification layer comprising a material selected from the groupconsisting of Ti, Ni, Cr, V, W, Re and a combination thereof, the atleast one electrode being in communication with the gas sensing layer,and wherein the gas sensing layer is capable of detecting at least onegas selected from the group consisting of NO, NO₂, SO_(x), O₂, H₂O, andNH₃.
 26. A method for making a gas sensor, the method comprising thesteps of: providing a substrate; disposing a heating layer adjacent tothe substrate layer; disposing a first glass layer adjacent to theheating layer; disposing a temperature sensing layer adjacent to thefirst glass layer; disposing a second glass layer adjacent to thetemperature sensing layer; disposing at least one electrode adjacent tothe second glass layer, disposing a titanium layer adjacent to the atleast one electrode; disposing a response modification layer adjacent tothe titanium layer; and disposing a gas sensing layer comprisingWO_(3-δ), wherein 0.35≧δ>0, and comprising a dopant selected from thegroup consisting of Re, Ni, Cr, V, W or a combination thereof, adjacentto the response modification layer.